1. Field of the Invention
The present invention relates to an output driver for a semiconductor device, and more particularly to an output driver for a semiconductor device with an improved slew rate characteristic.
2. Description of the Prior Art
Semiconductor devices such as DRAMs need an output driver having superior driving capability in order to transmit/receive data to/from exterior systems such as chip controllers. Generally, a slew rate of an output driver having superior driving capability has a lower limit value. That is, an output driver should be designed to have a slew rate exceeding the lower limit value regardless of variation of the process-voltage-temperature (hereinafter, referred to as “PVT”).
However, when a slew rate exceeds a predetermined level, the following problems may occur.
First, when a slew rate of an output driver is too high, amount of an instantaneous current consumed by the output driver may increase. Therefore, a driving voltage decreases due to inductance and resistance of a power line, and a ringing phenomenon may occur.
Secondly, as a slew rate increases, a reflective wave effect may increase due to incomplete termination in a transmission line connecting a semiconductor device, such as a DRAM, to an exterior system, thereby deteriorating signal integrity.
Because of these problems, it is also important that a slew rate of an output driver does not exceed a predetermined value. Therefore, it is preferred to design an output driver in such a manner that a slew rate has a value between an upper limit value and a lower limit value by considering the variation of the PVT.
FIG. 1 is a circuit diagram of a conventional output driver for maintaining variation of a slew rate, which is caused by variation of PVT, within a predetermined range.
Referring to FIG. 1, when a ‘/up’ signal is changed into a low level, a pre-driver 100 turns on a pull-up transistor 11, and when a ‘dn’ signal is turned to a high level, a pre-driver 110 turns on a pull-down transistor 12. Transistors 13 and 14 are connected in series between the pull-up transistor 11 and the pull-down transistor 12 and function as a factor of decreasing the value of the slew rate in the output driver. That is, with the prior art, it can be understood that resistors are provided to the output end of the output driver so as to prevent the slew rate from exceeding a predetermined value.
However, the prior art shown in FIG. 1 still has a problem in that the slew rate becomes unstable when the resistance values of the resistors are varied according to variation of the PVT.